1. Field of the Invention
This invention relates to a semiconductor device producing method, and more particularly to a thin film producing method using a chemical vapor deposition method utilizing light.
With the progress of semiconductor technologies, integration of semiconductor devices to get higher degree, as in the case of very large scale integrated circuit (LSI), is constantly sought for. Since the integration of higher degree can be realized by minimizing the circuit components, pattern forming technique of high precision and thin-film forming technique of high quality are urgently required.
A chemical vapor deposition method (CVD method) is well known in the art of forming thin film. According to this method, one or more compounds and/or a simple substance of 1n element forming the thin film is supplied in a gas state onto a substrate so as to grow a thin film of the element on the substrate, as a result of a chemical reaction. The CVD method can be further classified into an ordinary pressure CVD method, reduced pressure CVD method, plasma CVD method and the like. Since the ordinary pressure CVD method deposits a thin film under an ordinary pressure (760 Torr), the deposition speed thereof is high. However, a gas-phase reaction tends to occur during the deposition, and since it is difficult to heat the reaction chamber entirely, means for heating the substrate directly and evenly must be provided in the chamber. As a consequence, it is difficult to accommodate a large number of substrates in the chamber, thus lowering the productivity. Furthermore, the depositing temperature is difficult to control, and evenness of the thin film thereby obtained cannot be satisfactory. On the other hand, the reduced pressure CVD method deposits the thin film under a pressure of less than one Torr, and hence the depositing speed thereof is lower than that of the ordinary pressure CVD method. However, a large number of substrates can be accommodated in the chamber, so that the productivity of the reduced pressure CVD method is high.
Each of the above described methods utilizes a high-temperature heat energy for the reaction for the formation of the thin film. For instance, in a case of forming a polycrystalline silicon film by thermal decomposition of silane, a temperature of 600.degree.-800.degree. C. is utilized, and in a case where a thin film of silicon dioxide is formed by the thermal decomposition of tetraethyl-orthosilicate (TEOS, SiO.sub.4 C.sub.8 H.sub.2 O), a temperature of 650.degree.-800.degree. C. is used. For this reason, the thin film produced by these methods is restricted in its application in spite of the advantageous features they have. For instance, the silicon dioxide film formed by the TEOS thermal decomposition cannot be used for interlayer insulation film to be used for multilayer wiring using aluminum.
On the other hand, the plasma CVD method is a method wherein an electric energy is imparted, under a reduced pressure, to a reactive gas so as to provide plasma of chemically active molecules, atoms, ions, radicals and the like, and the chemical reaction of the gas is thereby promoted to accelerate formation of the thin film at a low temperature. However, when a silicon nitride (Si.sub.3 N.sub.4) film is produced by the plasma CVD method from a mixed gas of silane (SiH.sub.4)+nitrogen (N.sub.2), the compositions of the silicon (Si) and the nitrogen (N) tend to be deviated from the stoichiometrical values thus entailing disadvantages such as damaging the under layer by hydrogen radicals and deteriorating the film by seized hydrogen and thermal deformation.
For obviating the above problems, so-called photo CVD method which utilizes light energy to the formation of the thin film has been developed recently and is now studied intensely. According to one example of this method, the substrate is locally heated by use of a laser light of 10.6 .mu.m wave-length produced from, for instance, a CW carbon dioxide gas laser (CO.sub.2 laser ) of 50 W output, and a polycrystalline silicon film is formed on the substrate out of a material gas made of silicon-tetrachloride (SiCl.sub.4). Alternatively, a polycrystalline silicon film is formed on a substrate by way of light decomposition caused by an excimer laser using argon fluoride (ArF), krypon fluoride (KrF) and the like. Since these examples utilize a laser of large output as a spot light source, productivity thereof is not sufficient, and hence are not utilized practically.
In another example of the method, utilizing light energy, a light sensitization reaction is effected in a reactive gas containing a trace of mercury (Hg) by use of a low or high pressure type mercury lamp, thereby providing a film of silicon dioxide (SiO.sub.2) or silicon nitride (Si.sub.2 N.sub.4). In this case, added mercury (Hg) absorbs light of 2537 .ANG. wavelength emitted from the mercury lamp, and is brought into an excited condition of Hg* (3P.sub.1). Upon bombarding with the reactive gas, the absorbed energy is transferred, and the chemical reaction is thereby maintained. In this case, there is a problem that the heavy metal mercury becomes contained in the film.
As is apparent from the above description, the conventional photo CVD method utilizes high light energy of a short wavelength (.lambda.&lt;350 nm ) to perform light decomposition of a material gas or a large output light source of long wavelength to heat substrate.
Furthermore, a method for forming a refractory metal film of tungsten (W), molybdenum by reducing halogen compounds of refractory metals such as tungsten (W), molybdenum (Mo) and the like by use of hydrogen or silicon exhibits a strong under-layer dependency enabling to form the refractory metal film selectively on the silicon surface of a substrate having, for instance, silicon and silicon dioxide in a mixed manner on its surface, and hence is widely known as a selective CVD method.
More specifically, as shown in FIG. 1(a), when a chemical vapor deposition process is applied to a P type silicon substrate 103, on which are formed an operative region made of, for instance, N.sup.+ silicon diffusing layer 100 and a silicon dioxide film 102 having a contact hole 101, while using tungsten hexafluoride (WF.sub.6) as a reactive gas and hydrogen gas as a carrier gas (or a reduction agent), the tungsten hexafluoride is reduced in accordance with the following formulas (1) and (2), and a tungsten film 104 can be formed selectively on the silicon surface of the substrate as shown in FIG. 1(b). EQU WF.sub.6 (g)+3/2Si(S).fwdarw.W(S)+3/2 SiF.sub.4 (g) . . . (1) EQU WF.sub.6 (g)+3H.sub.2 (g).fwdarw.W(S)+6HF(g) . . . (2)
where g designates gas phase, and S designates solid phase.
The above described technique is attempted to be applied to various fields such as
(1) formation of intermediate layers at the time of providing electrodes in source-drain-gate regions for reducing the resistance of electrodes; PA0 (2) formation of barrier metal layers in contact holes for preventing the occurrence of boundary surface reactions between the semiconductor regions and the electrode layers; and PA0 (3) flattening the contact hole and through hole by way of deposition prior to the formation of the connection (wiring) layer for preventing disconnection of the connecting layer at stepped portions formed on the contact hole and through hole.
However, the technique has exhibited following problems.
An erosion phenomenon of silicon substrate tends to occur because of the chemical reaction of formula (1). This phenomenon essentially takes place in an isotropic manner so that the N.sup.+ silicon diffusion layer is eroded laterally and depthwisely at the same rate. However, the boundary layer between silicon and silicon dioxide is ordinarily more easily eroded, and in an extreme case, short circuiting between the electrodes and leakage due to a junction break-down tends to occur. Thus it has been required to control the reaction more precisely at the time of (1) formation of the intermediate layer and (2) formation of the barrier metal layer as miniaturization of the elements proceeds.
Furthermore, at the time of (3) flattening the contact hole and through hole by deposition, when the tungsten layer is made thick, tungsten grains 105 are deposited on the silicon dioxide layer, and the selectivity of the deposition is thereby impaired. In this case also, it is found that a precision control of the chemical reaction is required, and a process for removing the tungsten grains has been required additionally.